I have visited this river for two seasons as expedition member aboard Orion 2. We always had great views of different species of wildlife.
The Kinabatangan River is located in the Malayasian state of Sabah on the island of Borneo. Even though much of the vegetation has been cleared in the region, lots of wildlife remains in patches of forest along the river.
There is a range of birds that can be seen along the river. Most memorable on the smaller cruises are the kingfishers. There is also a range of birds of prey and hornbills to be seen on the transfers up the river.
The lower parts of the river support a population of crocodiles, including some very large individuals. Primates are common on the river, with regular views of Proboscis monkeys, and also the occasional orangutan.
If you are really lucky (I have been, twice), you might get to see the Borneon race of the Asian elephant.
Most of the lodges offer night time boat tours. You may not see a lot of different species, but the things you spot have the potential to be unique. Both times I went I saw interesting things, and it is a beautiful experience being on the river at night. It is one of the best ways to see Owls..
The main point of access is where the Kinabatangan River system ends, at the town of Sandakan. There are flights here from other parts of Malaysia, or you can drive from the capital of Sabah, Kota Kinabalu. The main way to access the river is through an organized tour. There are many companies that do multi-day tours; they include transfers, lodge accommodation, widlife tours and meals.